1. Field of the Invention
The present invention relates to methods and apparatus for recoiling metallic strips on a slitting line, and more particularly, to those methods and devices implementing steps or components for maintaining uniform tension in the strips as they are being recoiled.
2. Prior Art
Sheet metal produced by strip mills is typically sold in large, coiled rolls since the cost of producing sheet metal on a per pound basis varies inversely with the width of the sheet produced. Those purchasing these large coils and desiring to manufacture various products from the sheet metal find it necessary to have the sheet metal slit into narrow strips on a slitting line. The basic components of a slitting line are an uncoiler, on which is mounted the coiled sheet metal, a slitting machine having upper and lower rolls fitted with a plurality of knife edges which interact to slit the web from the coiled sheet metal into a plurality of strips, and a recoiler having a mandrel on which the slitted strips are recoiled into individual strip coils. In addition, a typical slitting line may contain a number of other components such as edge control devices to maintain the horizontal orientation of the web as it is being fed into the slitter, pinch rolls which facilitate the entry of the web into the slitting machine, and drag wipes for maintaining proper tensioning of the strips as they are taken up on the recoiler.
A problem inherent in the operation of all slitting lines is caused by the variation in thickness across the width of the sheet. Present sheet metal forming processes produce sheet metal that is thinner at its longitudinal edges than at its center, resulting in a "crowned" contour or slight convexity across its width. As sheet metal having such a crowned contour is slit and recoiled into separate strip coils on the recoiler, the strips formed from the portion of the web adjacent its longitudinal edges are thinner than those formed from the center of the web.
As the strips are rewound into coils, those strip coils formed from the thinner outer strips do not increase in diameter at the same rate as those strip coils formed from the thicker center strips. For a given number of mandrel revolutions, the diameter of a strip coil--made up of successive windings of thin strips--will be less than one made up of successive windings of thick strips. The result is that, for each revolution of the mandrel on the recoiler, a shorter length of a thin strip is being wound onto its respective strip coil than the length of thick center strip wound onto the center coils. Since the metal web is fed into and emerges from the slitter at the same rate across its width, this difference in recoiling rate results in a slackening of the thin strips from the slitter to the recoiler while the thicker strips remain properly tensioned. If a large coil of sheet metal is being slit on a slitting line, a considerable amount of slack strip is generated and results in unacceptably loose coils and possible fouling of the slitting line with damage to the material and danger to the personnel in the immediate area of the recoiler.
Many devices have been used in combination with a slitting line in order to compensate for the formation of slack strips between the slitter and the recoiler. For example, U.S. Pat. Nos. 3,771,738 and 3,685,711 disclose a tensioner having a pair of rollers used for maintaining tension between the tensioner and the recoiler, thereby insuring proper tensioning of the strips on the strip coils. However, this results in a slack strip condition between the device and the slitter. As a result, it is necessary to include a looping pit in combination with these tensioning devices so that the slack strips may drop down into the pit during the coiling operation. While the use of a looping pit and tensioning rolls is a simple and effecting expedient, the cost of excavating a looping pit is considerable and makes the relocation of the slitting line extremely impractical.
A second solution to the problem is disclosed in U.S. Pat. Nos. 4,093,140 and 2,726,051. These patents disclose slitting lines in which the strip coils are wound on individual drums mounted on the mandrel of a recoiler. The mandrel turns at a speed that exceends that of any of the individual drums which slip relative to the mandrel. This permits a variation in the rotational speeds of the drums and eliminates slack strips. An inherent disadvantage of such systems is the complexity and expense of the specially designed recoiler and drums. In addition, if strips of varying sizes are to be cut by the slitter, it is necessary to change the width of the drums on the recoiler mandrel making it necessary to maintain a stock of drums of varying widths.
Another solution consists of inserting slips of paper into the nip of a strip coil formed from the thinner strips. The addition of paper between successive windings of the selected strip coil increases the diameter of the strip coil for a given number of windings since the thickness of the paper is added to that of the strips. This process is continued until the diameter of the strip coil approximates that of a strip coil formed from the thicker center strips.
There are many disadvantages inherent in this method. Paper is difficult to insert in the proper location while the line is running. Stopping the operation to insert paper reduces production efficiency. In addition, any moisture contained in or subsequently absorbed by the paper would cause the strip it contacted to rust if the material slit was susceptible to rusting. There is also a danger of the paper combining with oil to stain the metal, which is undesirable when the metal is to be left unpainted and visible when incorporated with the finished product. In applications in which the paper strips are inserted by hand, there exists the danger that the person injecting the paper may become mangled by the recoiler.
Accordingly, it is desirable to operate a slitting line in a manner that eliminates the generation of slack strips between the slitter and the recoiler and does not require expensive looping pits, recoilers with separate drums, or the use of paper strips which increase cost and mar the product.